Method for treating aqueous sludge, material so produced and the use thereof

ABSTRACT

The invention relates to a method for the accelerated dehydration of sludges in sludge disposal areas, especially of sludges from rivers and harbors, by the exclusive use of anionic polymer flocculants and the use of materials obtained according to the inventive method.

The present invention relates to a method for accelerated dewatering ofsludges in silt fields, especially of sludges from rivers and harbors,by the use of polymeric flocculating agents, and to the use of materialobtained by this method.

Inorganic and organic sediments are constantly transported downstream byriver currents. These sediments accumulate in the rivers and harbors. InGB 1116290 it is proposed that polymer solutions be flushed into deeperlayers of these sediments in order to initiate flocculation processesthat on the whole lead to loosening of the deposits, so that they can becarried away by water currents. Polyacrylic acid and polyacrylamideamong other compounds are cited as polymers. Copolymers contain at least50 mol % of acrylic acid or methacrylic acid. Furthermore, cationicpolymers are also described as suitable.

Modern practice, however, is to remove these sediments from the water bydredgers. The sediments frequently contain environmentally hazardousconstituents in the form, for example, of complexed heavy metal ions orhazardous organic substances, and so it is no longer permissible to dumpthem in deeper waters, as was frequently done in the past. Instead, theymust be consigned to ultimate storage under environmentally safeconditions on land.

To permit proper ultimate storage, the sediments, which may contain upto 20 wt % of organic constituents depending on their origin, must beappropriately pretreated. In the method now being used in practice, thedredged sediment sludges are transported in barges to the facilitiesprovided for sludge treatment and flushed at rates of 1000 to 6000 m³/hthrough pipelines to appropriate dewatering fields. Dewatering of thesludges takes place during sedimentation by seepage into drains, bydraining off the supernatant water formed during sedimentation and bynatural drying. After a semi-solid consistency has been reached, dryingof the sludge is continued by multiple mechanical turning (DE 19726899A1; Heinrich Girdes GmbH, 1998).

The additional input of water due to weather influences leads toremoistening of the sludges and in this way slows the drying operation.Depending on location, rainfall can cancel out evaporative drying in asmany as 8 months of the year. The entire process of sludge conditioningneeds up to one year and can be greatly prolonged up to 18 months byhigher contents of fine-grained sludge fractions in the size range of0.06 mm and smaller, since deposits thereof form sediment layers thatare almost impervious for water and block seepage through drainagedevices (see also Prof. Fritz Gehbauer, Institute for MechanicalEngineering in Construction, Fridericiana University, ImbVeröffentlichung, Series V, No. 20, Floating Dredger Technology, Chapter3.2. Definitions, page 29). Because of the lower density of fine-grainedsludges, the dewatering basins hold less dry substance for the samefilling level, meaning that the sludge throughput is reduced comparedwith coarse-grained sludges. To achieve sufficient vane shear strength,which is necessary for further processing of dried sludge, thefine-grained sludge must be dried to a water content of 60 wt %, whereascoarse-grained sludge already meets the strength requirements at 65 to70 wt %.

In U.S. Pat. No. 3,312,070 (Daiichi Kogyo Seiyaku Kabushiki Kaisha,1967), the use of surface-active auxiliaries having a coagulating effectis proposed for recovery of sludges, which without these auxiliariestend to separate into fine and coarse fractions. This in turn results indifferent material properties of the recovered sludges. Among otherexamples in the patent, reaction products of acrylamide andcarboxymethylcellulose, polyacrylamide, polyvinyl alcohol, mixtures ofpolyacrylamide with aniline-urea-formaldehyde resins and sulfomethylatedpolyacrylamide are used. The auxiliaries are metered into the feed linethat transports the sludges to the settling basins.

In EP 346159 A1 (Aoki Corp., 1989), it is stated that the method ofconventional sludge dewatering, in which the negatively charged sludgeparticles are treated with cationic salts or cationic polymers, isdisadvantageous in terms of flocculation effectiveness and costs. As analternative, the successive addition of an anionic and a cationicpolymeric coagulating agent and if necessary of a further anionicflocculating agent is proposed for sludge dewatering. Because of thepractical circumstances, whereby the flocculating agent is mixed in withthe rapid stream of aqueous sludge before it enters the sludge settlingbasin, it is not guaranteed that two or three different flocculatingagents matched to one another can be successfully metered in so thatthey will interact to form sedimented sludge flocs.

From EP 0500199 B1 (Detlef Hegemann GmbH & Co., 1996) there is known amethod for conditioning of the contaminated sediments from bodies ofwater to obtain a building material that is durably safe for theenvironment, wherein the sediments are formed to an environmentally safebuilding material by means of clay materials and cement/slaked limeafter they have been dried to a water content of approximately 120 to140%.

The object of the present invention is to provide, for dewatering ofsludges from rivers and harbors and sandbanks or sea floors, a methodthat permits rapid and cost-effective dewatering and that can beachieved as simply as possible in the existing field-dewatering systems.In view of the high velocities of transport of the sludges and theassociated high shear loads, it is intended that the method to beprovided will lead to rapid flocculation and stable flocs. Inparticular, it is intended that the method will be advantageously usableeven for sludges that are particularly difficult to dewater because oftheir content of fine particles.

As regards performing the method, an important aspect, with a view tosubsequent use of the dried sludges, is that environmentally hazardousproducts be avoided.

Another object of the method to be provided is to minimize the time fornatural drying of the partly dewatered sludge remaining after separationof the supernatant water and of the drainage water. A further object ofthe invention is to bind the hazardous substances present in the aqueoussludges so strongly to the dewatered sludges that subsequent addition ofsubstances for binding hazardous substances can be reduced or avoidedand the dried sludges can be directly further processed or dumped.

The object is achieved according to the invention by a method fordewatering of sludge wherein the sludge

-   is adjusted to a pumpable concentration by addition of water-   is flushed through a pipeline to a dewatering field-   is mixed with an aqueous solution of a polymeric flocculating agent    while it is being transported-   is sedimented in the dewatering field and partly freed of    supernatant and/or drainage water and then subjected to natural    evaporative drying

characterized in that the flocculation is achieved with a water-soluble,anionic, polymeric flocculating agent.

The anionic polymers to be used according to the invention have aweight-average molecular weight Mw of higher than 1.0×10⁷, preferablyhigher than 1.2×10⁷ and particularly preferably higher than 1.5×10⁷.

The anionic polymers to be used according to the invention are formedfrom water-soluble, nonionic and anionic monomer constituents. Examplesof usable nonionic monomers are acrylamide and methacrylamide, thehydroxyalkyl esters of acrylic and methacrylic acid, preferably2-hydroxyethyl and 2-hydroxypropyl esters, acrylonitrile,vinylpyrrolidone and N-vinylacetamide and mixtures thereof. Preferablyacrylamide is used.

Examples of usable anionic monomers are unsaturated monocarboxylic anddicarboxylic acids, such as acrylic acid, methacrylic acid, itaconicacid, maleic acid, fumaric acid, vinylsulfonic acid,acrylamidoalkanesulfonic acids, vinylphosphonic acid and/or their saltswith alkalis, ammonia, (alkyl)amines or alkanolamines and mixtures ofthese monomers. Preferably acrylic acid and its alkali salts are used.

For modification of the polymer properties, the polymers to be usedaccording to the invention can contain up to 10 wt % of further monomersthat are insoluble or slightly soluble in water, provided they do notimpair the water solubility of the polymer. Examples of such monomersare vinyl acetate and alkyl acrylates, such as methyl acrylate, ethylacrylate, butyl acrylate, ethylhexyl acrylate.

The content of anionic monomer constituents in the anionic polymers tobe used according to the invention ranges from 1 to 40 wt %, preferably5 to 30 wt % and particularly preferably 10 to 20 wt %.

As regards the somewhat coarser sludges, it has been found that in somecases an anionic polyacrylamide with an anionic content of 30 to 40 wt %has a particularly good flocculating effect.

In a further advantageous embodiment of the invention, there is used amixture of two different anionic flocculating agents, wherein thedifferences can be both in the chemical structure of the anionic monomercomponents and in the proportions by weight of the anionic monomercomponents. The use of a mixture of polymers having differentproportions by weight of anionic monomer constituents is preferred.

As an alternative to the polymers synthesized from nonionic and anionicmonomers, there can also be used polymers that were originally formedfrom nonionic monomer constituents and in which anionic groups aregenerated by partial hydrolysis, for example of ester-like and/oramide-like nonionic constituents. An example in this regard is anionicpolyacrylamide generated by hydrolysis of polyacrylamide homopolymer.

The anionic polyacrylamides can be produced by various polymerizationmethods, an example being the gel polymerization method, in which theaqueous monomer solution, after initiation, is polymerized adiabaticallyto a gel, which is then subjected to size reduction, drying and grindingto polymer powder. To improve the dissolution behavior, the polymerpowders are often coated at the surface, for example with finely dividedaerosils or else with water glass. Furthermore, the polymers can alsoexist in the form of a water-in-oil emulsion, which is inverted to anoil-in-water emulsion before application. Preferably an aqueous solutionof powdery polymers is used.

The aqueous polymer solution is produced by dissolving or dilutingpowdery polymers or emulsified polymers and is added as a dilutesolution. The concentration of the polymer solution is usually lowerthan 2 wt %, preferably lower than 1% and particularly preferably lowerthan 0.5 wt %.

The sludge dredged from rivers and harbors, for example, or from the seafloor or sandbanks. is adjusted with water to a pumpable concentration,preferably to a density of 1.04 to 1.15 metric tons per m³ andtransported through pipelines to the sludge-dewatering fields. Ameasuring device that determines the instantaneous sludge concentrationis mounted in the pipelines. By means of the measurement signal thenecessary quantity of flocculating agent is calculated and metering ofthe flocculating agent is initiated.

The anionic polymers are added to the aqueous sludges as aqueoussolution, addition taking place in the feed line transporting theaqueous sludge to the dewatering basin. Intimate mixing of the polymersolution with the sludge is achieved by the turbulence in the flowingslurry and can be assisted if necessary by mixing elements, preferablystatic mixing elements. The point of addition of the polymer solutioninto the sludge feed line can be chosen such that the flocculationprocess has already begun or is just beginning at the inlet to thedewatering basin.

Surprisingly, it has been found that the flocs produced according to theinventive method are very stable, and thereby destruction of alreadyformed flocs by friction effects during sludge transport is prevented.By virtue of the high stability of the flocs produced by the inventivemethod, the point of addition of the solution of flocculation agent intothe sludge flow can also be located far from the inlet to the settlingbasin. Because sludge flocculation begins rapidly, it is also possibleto meter the flocculation agent in right at the inlet to the settlingbasin. In a preferred embodiment, metering of the flocculating agenttakes place over a short mixing section ahead of the inlet, especiallyless than 20 m therefrom.

The quantity of anionic polymer necessary for the inventive methoddepends on the concentration and composition of the sludges, and it canbe determined by simple preliminary experiments. Relative to the solidscontent of the sludge, the proportion of polymeric flocculating agentadded usually ranges from 0.02 to 2 wt %, preferably 0.05 to 1 wt % andparticularly preferably 0.09 to 0.5 wt %.

The achievement of the object by the inventive method is surprisinginsofar as it permits flocculation and sedimentation of the negativelycharged sludge particles by the use of anionic flocculating agentsalone, without addition of further flocculation auxiliaries. Inparticular, sludges containing fines can be advantageously processed,since the floc structure formed no longer prevents blocking of thedrainage dewatering.

The time advantages achieved by the inventive method are considerable.In the conventional treatment without flocculation auxiliary, andstarting from a raw density of approximately 1.04 to 1.15 metric tonsper m³ for the sludge to be subjected to the treatment, the supernatantwater can be drained off after approximately 2 to 7 weeks in the case ofcoarse sludges and after approximately 4 months in the case of finesludges. At that time the sludge has a density of approximately 1.16metric tons per m³ in the case of fine-grained sludges and 1.22 metrictons per m³ in the case of coarser sludges. Fine-grained sludges withinthe meaning of the present disclosure are sludges whose content ofparticles in the size range of 0.06 mm and smaller is at least 50 wt %.

At this time, mechanical turning is begun, in order to promote naturaldrying by evaporation. Depending on season and weather conditions, thisoperation is ended after a few months, at which time the sludge has adensity of approximately 1.47 metric tons per m³ and can be sent tofinal use. In the inventive method, the supernatant water can already bedrained off within the first 24 hours, and this improved drainagedewatering is accompanied by a much drier sludge. At this time thesludge already has a density of 1.25 to 1.35 metric tons per m³ and thencan either be sent directly to further processing or dried moreintensively by evaporative drying, if necessary coupled with mechanicalworking. Densities of higher than 1.45 metric tons per m³ and preferablyof 1.47 metric tons per m³ or higher are usually achieved afterevaporative drying. The dried sludges produced in the inventive methodhave vane shear strengths of greater than 25 kN/m², preferably ofgreater than 30 kN/m².

The mechanical working of the sludge leads to regular breaking of thesludge surface, whereby evaporative drying is accelerated. This workingcan be applied by excavators, for example, which turn the sludgeregularly, or else by rotary hoes, which work the surface of the sludgevery intensively. According to the invention, rotary hoes are preferablyused.

In the inventive method, the water separated from the surface and bydrainage in the dewatering field is returned to the sludge-transferpoint via closed circular pipelines and reused for dilution of thedelivered sludge.

Surprisingly, the partly dewatered sludge obtained after separation ofsupernatant and drainage water exhibits only a slight tendency toreabsorb newly arriving water. By virtue of this property, theweather-related prolongation of the natural sludge drying is greatlyreduced, since rainwater, for example, now collects mainly in thesurface region of the sedimented sludge and no longer remoistens orslurries the sludge.

After the sludges have reached the desired degree of dryness, they aredeposited in dumps or processed to an environmentally safe buildingmaterial, for example to a sealing material, to edge fill or to pavementstructures. Products manufactured according to the inventive method haveadvantageous material properties.

Since the hazardous substances present in the aqueous sludge areincorporated into the flocs formed in the inventive method, the additionof agents that bind hazardous substances can be reduced or evendispensed with. Thus the sludges can be directly dumped or furtherprocessed to building materials once they have been dried.

The use of anionic polyacrylamides does not cause any environmentallyhazardous burden of the sludge, and so further processing free ofproblems is possible. In the further processing of highly contaminatedsludges to an environmentally safe building material, the addition ofmineral fillers, especially of clays, slaked lime and cement or mixturesthereof has proved advantageous. In particular, 1 to 15 wt % of each ofthese substances is mixed homogeneously into the sludge.

Determination Methods

Dewatering by the Screen-Test Method

By means of this method, polymeric flocculating agents are tested withregard to their suitability for conditioning and dewatering of sludges.

In a 700-ml graduated beaker, 500 ml of sludge is mixed with the 0.25%flocculating-agent solution to be tested and stirred with a 4-fingerstirrer at 1000 rpm for a certain time. After this conditioning, thesludge sample is filtered (=dewatered) on a metal screen (200 micrometermesh openings). The dewatering time for a filtrate volume of 200 ml ismeasured, and the clarity of the collected filtrate is evaluatedoptically in a clarity wedge.

-   Clarity “0”=no clarification-   Clarity “46”=best clarification    Sedimentation Experiment

In a 600-ml glass beaker, 500 ml of mud and a certain volume of 0.25%flocculating-agent solution are stirred with a 4-finger stirrer at 1000rpm for 10 seconds. The volume (ml) of clarified water formed above thesediment is evaluated as a function of sedimentation time (minutes).

Vane Shear Strength

The shear strength of the dewatered sludge is measured with a vaneprobe. The vane probe is composed of a rod, at the bottom end of whichthere are disposed 4 vanes. The dimensions and mode of operation arestandardized according to DIN 4096. To perform the experiment, the vaneprobe is pressed into the sludge and turned slowly until the sludgebreaks along a cylindrical sliding surface. The vane shear strength iscalculated from the torque M measured at break and the diameter d of thevane probe.FS=6×M/7×π×d ³

EXAMPLES

Polymers Used

The anionic polymers used all had an average molecular weight Mw ofhigher than 15 million; the cationic polymers had an Mw of higher than 6million. All polymers were used in the form of their 0.25 wt %solutions.

Polymer A: cationic polyacrylamide with a 25 wt % content of quaternizeddimethylaminopropylacrylamide

Polymer B: cationic polyacrylamide with a 10 wt % content of quaternizeddimethylaminopropylacrylamide

Polymer C: cationic polyacrylamide with a 6 wt % content of quaternizeddimethylaminopropylacrylamide

Polymer D: anionic polyacrylamide with a 1.5 wt % content of acrylicacid sodium salt

Polymer E: anionic polyacrylamide with a 10 wt % content of acrylic acidsodium salt

Polymer F: anionic polyacrylamide with a 15 wt % content of acrylic acidsodium salt

Polymer G: anionic polyacrylamide with a 40 wt % content of acrylic acidsodium salt

Sludge Sample 1

For this there was used harbor mud from Bremen with a concentration of10.3% of dry substance, corresponding to a density of 1.066 metric tonsper m³. The average particle size was Dm=0.0564 mm, determined from theparticle-size line in the diagram obtained by the semilogarithmic plotof log particle diameter versus particle content by mass. The content ofparticles smaller than 0.06 mm was 70 wt %. The particle charge wasnegative and was determined with the PCD 03 pH meter of the Mutek Co.(from Herrsching, Germany) by polyelectrolyte titration to theisoelectric point. The measured value was −230 mV. The ignition loss ofthe dry substance was 13.2%, determined at 600° C./2.5 h.

Sludge Sample 2

For this there was used harbor mud from Bremerhaven with a concentrationof 9.5% of dry substance, corresponding to a density of 1.062 metrictons per m³. The average particle size was Dm=0.0212 mm, determined fromthe particle-size line in the diagram obtained by the semilogarithmicplot of log particle diameter versus particle content by mass. Thecontent of particles smaller than 0.06 mm was 100 wt %. The particlecharge was negative and was determined with the PCD 03 pH meter of theMütek Co. (from Herrsching, Germany) by polyelectrolyte titration to theisoelectric point. The measured value was −410 mV. The ignition loss ofthe dry substance was 15.5%, determined at 600° C./2.5 h.

Example 1

500 ml of sludge sample 2 was mixed with 40 ml of polymer solution inthe sedimentation experiment and sheared for 10 sec. After asedimentation time of 1 minute, the following volumes of clarified waterwere measured: Polymer Volume [ml] of clarified water B 5 C 20 D 210 E240 F 260 G 25 none 0

When the experiment was repeated with increased shearing (sheared for 30sec), the following volumes of clarified water were measured aftersedimentation times of 1 and 2 minutes: Volume [ml] of clarified waterPolymer after 1 minute after 2 minutes B 5 10 E 260 270

Example 2

500 ml of sludge sample 2 was mixed with 80 ml of polymer solution inthe sedimentation experiment and sheared for 10 sec. After asedimentation time of 1 minute, the following volumes of clarified waterwere measured: Polymer Volume [ml] of clarified water A 180 D 270 E 260F 220 G 10 none 0

Example 3

In a dewatering experiment by the screen-test method, the followingresults were obtained with sludge sample 1 and added polymer in aconcentration of 100 g of dry substance per m³ of sludge: PolymerClarity wedge Time [sec] for 200 ml of filtrate A 46 167 B 46 52 D 46 18E 46 6 F 46 6 G 46 11

When the added flocculating agent was increased to 300 g of drysubstance per m³ of sludge, the results were as follows: Polymer Claritywedge Time [sec] for 200 ml of filtrate D 31 5 E 27 15 F 22 31 G 21 102

Example 4

500 ml of sludge sample 1 was mixed with 40 ml of polymer solution inthe sedimentation experiment and sheared for 10 sec. After asedimentation time of 1 minute, the following volumes of clarified waterwere measured: Polymer Volume [ml] of clarified water B 60 D 140 E 150 F150 G 160 none 0

At increased shearing (30 sec stirring time), the flocculation resultsachieved according to the invention proved to be extremely stable:Polymer Volume [ml] of clarified water B 10 E 160

Example 5

500 ml of sludge sample 1 was mixed with 80 ml of polymer solution inthe sedimentation experiment and sheared for 10 sec. After asedimentation time of 1 minute, the following volumes of clarified waterwere measured: Polymer Volume [ml] of clarified water B 100 D 190 E 190F 180 G 130 none 0

Example 6

500 ml of sand was flushed into a vertical plexiglass tube of 80 mmdiameter and 500 mm length, covered at the bottom end with a 200 μmscreen.

1000 ml of fine-grained dredged sludge from Bremerhaven with a solidscontent of 15.3% and a density of 1.10 metric tons per m³ was mixed with160 ml of a 0.25% solution of polymer E in polder water and conditionedat 1000 rpm for 10 sec with a 4-finger stirrer.

1000 ml of this mixture was filled into the plexiglass tube and thedewatering behavior as well as the sludge density was determined:Experiment duration [h] 1 2 4 7 24 Filtrate [g] 616 672 702 727 739Density [metric 1.20 1.23 1.25 1.26 1.27 tons per m³]

1. A method for the dewatering of sludge comprising adjusting theconcentration of the sludge to a pumpable concentration by addition ofwater, flushing the sludge through a pipeline to a dewatering fields,mixing the sludge with an aqueous solution of a polymeric flocculatingagent while the sludge is being transported, allowing the sludge tosettle in the dewatering field to form a sediment and partly freeing thesludge of supernatant and/or drainage water and then subjecting thesludge to natural evaporative drying, wherein the polymeric flocculatingagent is a water-soluble, anionic, polymeric flocculating agent.
 2. Themethod according to claim 1, wherein the anionic polymeric flocculatingagent is formed from anionic wherein nonionic monomers and acrylic acid,methacrylic acid, itaconic acid, maleic acid, fumaric acid,vinylsulfonic acid, acrylamidoalkanesulfonic acids, vinylphosphonic acidand/or their salts with alkalis, ammonia, (alkyl)amines or alkanolaminesor mixtures of these monomers are used as the anionic monomers andwherein acrylamide, methacrylamide, acrylonitrile, hydroxyalkyl estersof acrylic and methacrylic acid, vinylpyrrolidone or vinylacetamide ormixtures of these monomers are used as the nonionic monomers.
 3. Themethod according to claim 1, wherein a polyacrylamide formed frompolymerized acrylamide and acrylic acid units is used as the polymericflocculating agent.
 4. The method according to claim 1, wherein thepolymeric flocculating agent contains 1 to 40 wt % of integrallypolymerized anionic monomer constituents.
 5. The method according toclaim 1, wherein the polymeric flocculating agents have a weight-averagemolecular weight Mw of higher than 1.0×10⁷.
 6. The method according toclaim 1, wherein at least two different anionic flocculating agents areused.
 7. The method according to claim 1, wherein the polymericflocculating agent is added in a proportion of 0.02 wt % to 2 wt %relative to the solids content of the sludge.
 8. The method according toclaim 1, wherein the polymeric flocculating agent is used in the form ofan aqueous solution with a concentration of lower than 2 wt %.
 9. Themethod according to claim 8, wherein the polymer solution is preparedfrom a powdery polymer.
 10. The method according to claim 1, wherein thesludge to be treated was obtained from rivers, harbors, the sea floor orsandbanks.
 11. The method according to claim 1, wherein the sludge to bedewatered contains at least 50 wt % of fine particles in the size rangeof 0.06 mm or smaller.
 12. The method according to claim 1, wherein thesludge to be dewatered is adjusted to a density of 1.04 to 1.15 metrictons per m³ by addition of water.
 13. The method according to claim 1,wherein the flocculating agent is metered into the pipeline over asection between the outlet to the dewatering field and 150 m ahead ofthe outlet.
 14. The method according to claim 1, wherein a measuringdevice in the pipeline determines the sludge concentration, calculatesthe quantity of flocculating agent therefrom and initiates metering ofthe flocculating-agent solution.
 15. The method according to claim 1,wherein the sludge treated with the flocculating agent has a density of1.25 to 1.35 metric tons per m³ after dewatering and before naturalevaporative drying.
 16. The method according to claim 1, wherein thenatural evaporative drying is accelerated by mechanically turning thesludge.
 17. The method according to claim 16, wherein the mechanicalturning is achieved by means of rotary hoes.
 18. The method according toclaim 1, wherein the evaporative drying of the sludge is continued to adensity of at least 1.45 metric tons per m³.
 19. The method according toclaim 18, wherein the sludge has a vane shear strength of greater than25 kN/m².
 20. The method according to claim 1, wherein the dewatered anddried sludge is mixed with clays and/or slaked lime and/or cement inproportions of 1 to 15 wt % each.
 21. A dewatered sludge preparedaccording to the method as claimed in claim
 1. 22. A building materialcomprising the dewatered sludge prepared according to the method asclaimed in claim 1.